The planes of Existence

Literally the world, but not to be confused with The World. Not much to say about it except that it's a Texture, a Layer of the fabric of reality that with several other layers make up the surface of The World

It is the layer of the World that retains the laws of the Age of the Gods, housing surviving Phantasmal species instead of people. Also known as the Inner Sea of the Planet or the D-Layer it is the place where the soul of the World is kept.

It is the bottom Texture or Layer of the World located 2700 kilometers below the surface (digging won't get you there unless you can rip space-time), it is akin to several Reality Marbles merged together all representing a facet of The World.

The Reverse Side of the World is a legendary, otherworldly paradise that cannot be reached by normal ways, spoken of in many cultures around the world as a land of immortality. It has had many names throughout history, including Utopia, the Garden of Eden , Tír na nÓg, and Avalon.

It is not easy to reach the reverse side, you normally need very powerful magic, or just coincidence, or being dragged there, also not having a body is advisable due to the nature of the reverse side.

Swirl of the Root also known as The Root or The Akashic Records (simply Akasha)  is a metaphysical location that acts as the "force" that exists at the top of all theories in every dimension, as well as the source of all events and phenomena in the universe.

the "Outside of the World," as opposed to the World and its Reverse Side. Existing outside of time, it stores and archives information of all possibilities and events, past, present, and future, of the world. It is the place from where all souls including those of Heroic Spirits recorded on the Throne of Heroes, originate from and to where they return after death, before being reincarnated into something else.

Reaching The Root is one of the most difficult feats to achieve, the methods used to reach it usually involve reaching the pinnacle of a certain "concept". Those who have touched it directly have never returned to the world.

Also occasionally referred to as the Ring of Deterrence a place existing outside of both the World and the time axis. It is whereHeroic Spirits and divine spirits are stored.

It is implied by several events in FGO that although the throne exists outside of time and space, it has a space an time of its own as several servants have done things or met others in The throne. (This info is completely irrelevant to the campaign as your servant comes in blank memory of the throne but it is weirdly funny)

The Imaginary number space, the "anti-reality" is an alternative space within the Reverse Side of the World from which the Magecraft concept of Imaginary Numbers originates. It is an unobservable territory, undetectable by the current laws of physics. It contains all possibilities due to being unobserved, though for that same reason nothing actually exists within it.

It is not normally hostile to normal reality, which are known as "Real Numbers", but if the Imaginary Numbers were made observable (without any quantum shenanigans), then they could become lethal to normal life and the interaction between Real Numbers and Imaginary Numbers becoming observable to each other can cause insanity.

Anyone who studied magecraft knows this, whether from the Mage's Association, the church, or a standalone guy.
Nobody knows this.